A molecular dynamics simulation of collisional excitation mechanisms in Al

Abstract

A modified version of the SPUT2 molecular dynamics sputtering code was used to reinvestigate core excitation in Al atoms following bombardment with 1–5 keV Ar+ ions. For all bombarding energies, asymmetric collisions between the incoming ion and target atoms yielded smaller minimum distances-of-closest-approach between the collision partners for hard collisions than did symmetric collisions between pairs of target atoms. Simple critical distance-of-closest-approach models were used to estimate core excitation for both asymmetric and symmetric collisions. A single value of Rc (0.367 Å) was used for asymmetric Ar-Al collisions, while two choices of Rc were used for symmetric Al-Al collisions (0.442 and 0.530 Å). With the smaller Rc value for Al-Al collisions, we find that core excitation proceeds predominantly by asymmetric collisions at all bombarding energies above threshold. At 5 keV bombarding energy the percentage of sputtered, core-excited atoms originating from asymmetric collisions ranged from 89 to 95% depending on the incident direction of the projectile. With the larger Rc value, core excitation proceeds predominantly by asymmetric collisions at bombarding energies above approximately 3 keV; and at 5 keV asymmetric collisions accounted for ∼ 60 to ∼ 84% of sputtered, core-excited atoms. Lifetime corrections and corrections for Auger neutralization near the target surface had little effect on the ratio of asymmetric to symmetric collisions responsible for atomic-like Auger emission. These simulation results suggest that simultaneous multiple collisions are very important in the initial energy- and momentum-transfer stage which initiates the cascade.

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